# 9: Momentum, Force and Energy Flashcards

What is Newton’s first law?

the law of inertia. This states that a body at rest will continue at rest, or a body in motion will continue in constant motion, unless acted upon by an external force.

What is Newton’s second law?

the acceleration of an object is directly proportional to the force applied (F = ma)

(The rate of change of momentum of an object is directly proportional to the force which acts on the object)

Do all objects accelerate towards the ground at the same rate?

Yes in theory however air resistance (drag) means they don’t

What is Newton’s third law?

every action has a complete and opposite reaction

Where does Newton’s third law come from?

It is a consequence of the conservation of momentum. The resultant force acting means a change in mass or acceleration – which means a change in momentum.

Momentum is always conserved when no external forces act, so whenever one object exerts a force on another, the second object must exert equal size force back on the first object so that the overall change in momentum is zero

What is a perfectly elastic collision?

One where momentum is conserved and kinetic energy is conserved

What is an inelastic collision ?

One where some of the kinetic energy is converted into other forms during the collision. But momentum is always conserved

What are the two types of friction? What are the differences?

Contact friction between solid surfaces

Fluid friction, drag or fluid resistance or air resistance

Three things you need to know about or frictional forces:

Their direction?

Their affect on the speed of the object?

Which types of energy do they convert?

They always act in the opposite direction to the motion of the object

They can never speed things up or start something moving

They convert kinetic energy into heat

You will reach your terminal velocity at some point, if you have…

A driving force that stays the same all the time

A fictional or drag force that increases with speed

When does something reach terminal velocity?

When the frictional force equals the driving force

Describe the graph of velocity against time for an object reaching terminal velocity

Curve, where the gradient decreases, eventually turning into a horizontal line. Graph starts at the origin and increases

Describe the graph of acceleration against time for an object reaching terminal velocity

Acceleration starts off high then decreases. The rate of decrease of acceleration starts slow then speeds up then slows again. The acceleration finally reaches zero. The graph looks like an unexaggerated S shape

Describe the graph of the velocity against time for the parachutist

First half of the graph looks like the normal velocity against time for an object reaching terminal velocity. Then the graph suddenly drops to a lower velocity and flattens out, where it reaches a new terminal velocity

When is work done?

Whenever energy is transferred